A method for producing a shaped article by a selective laser sintering (SLS) process is disclosed in U.S. Pat. Nos. 4,863,538 to Deckard; 4,938,816 to Beaman et al; and, 4,944,817 to Bourell et al, the disclosure of each of which nis incorporated by reference thereto as if fully set forth herein. The shaped article is formed by sintering a powder of one or more materials. "Sintering" is defined by the heating of the powder to a temperature which causes viscous flow only at contiguous boundaries of its particles, with at least some portion of substantially all particles remaining solid. Such sintering causes coalescence of particles into a sintered solid mass the density of which is increased compared to the bulk density of the powder particles before they were sintered; and, a part formed by layer-wise joining of plural vertically contiguous layers is therefore said to be autogenously densified.
The goal of the invention is to produce a powder of partially coated particles no greater than 177 .mu. (microns, about 80 mesh U.S. Standard sieve series), which coated powder has a higher bulk density ("densified") than in the uncoated state, is surprisingly free from static electric charge, and is flowable at a temperature above its initial transition temperature at which it becomes. sticky, and would not be flowable without being coated. By "flowable" is meant that the partially coated powder can be transferred by spreading it, with essentially all particles of the powder being separate one from the other, in a free-flowing layer of uniform thickness, no more than about 250 .mu. (10 mils) thick, with the application of minimal force, less than 500 g-force, and without the partially coated powder (hereafter also referred to as "coated powder" for brevity) changing its particulate characteristics.
The uncoated "virgin" powder, referred to as the "host" or "first" powder, is coated with a coating which itself is also a powder and is referred to as the "coating powder" or "anti-caking" or "second" powder. The coating powder is made up of particles &lt;44 .mu. in average diameter, and smaller than those of the host powder. The size of both, the host particles and the coating particles is critical because when the host particles are &gt;177 .mu., most materials likely to be used in the process of this invention do not sinter at a high enough rate to form a sufficiently dense sintered body for the purpose at hand; and, coating particles 20 .mu. in average diameter, or larger, are reluctant to adhere to host particles of usable size. This is particularly true for a host powder of polycarbonate ("PC"), acrylonitrile-butadiene-styrene ("ABS"), or, poly(vinyl chloride) ("PVC") powder coated with an inorganic powder.
The coated powder is said to be flowable because individual particles do not coalesce (stick together). Contiguous particles coalesce when they reach a temperature at which each becomes soft enough to cause fusion. The property of non-coalescence of the novel powder is noteworthy because the property persists under temperature conditions at which one, knowing the uncoated host powder would coalesce, is surprised to find that the partially coated powder does not. The individual particles of coated powder are referred to as being "mottled" because the surface of each host particle is only partially coated with particles of coating powder, and not encapsulated.
Spreading of the coated powder is accomplished by exertion of only as little as 5 g-force, and no more than 500 g-force. The force is measured by a "Draw-Plate" technique described in greater detail in Polymer Engineering and Science, Vol. 28, 249 (1988).
The invention is also directed to a method of producing a solid porous article from the coated powder. This article not only has the precise dimensions of the shape desired, but is dense enough to provide a prototype having nearly all but those properties attributable to the fully dense (so termed if it has essentially no measurable porosity) article eventually to be manufactured.
The above method is carried out in an apparatus which includes a laser or other directed energy source which is selectable for emitting a beam in a target area where the part is produced. A powder dispenser system deposits powder into the target area. A laser control mechanism operates to move the aim of the laser beam and modulates the laser to selectively sinter only the powder disposed within defined boundaries to produce the desired layer of the part. The control mechanism operates selectively to sinter sequential layers of powder, producing a completed part comprising a plurality of layers sintered together. The defined boundaries of each layer correspond to respective cross-sectional regions of the part. Preferably, the control mechanism includes a computer--e.g. a CAD/CAM system to determine the defined boundaries for each layer. That is, given the overall dimensions and configuration of the part, the computer determines the defined boundaries for each layer and operates the laser control mechanism in accordance with the defined boundaries for each layer. Alternatively, the computer can be initially programmed with the defined boundaries for each layer.
A part is produced by depositing a first portion of powder onto a target surface, scanning the aim of a directed energy beam (preferably a laser) over the target surface, and sintering a first layer of the first powder portion on the target surface. The first layer corresponds to a first cross-sectional region of the part. The powder is laser-sintered by operating the laser beam when the aim of the beam is within the boundaries defining the first layers. A second portion of powder is deposited onto the first laser-sintered layer and the aim of the laser beam scanned over the first laser-sintered layer. A third layer of the powder is laser-sintered by operating the laser beam when the aim of the beam is within the boundaries defining the second layer. Sintering of the third layer also joins the first and second layers into a cohesive mass. Successive portions of powder are deposited onto the previously sintered layers, each layer being laser-sintered in turn.
Repetition of the foregoing steps results in the formation of a bed of powder, the surface of which continually presents the target surface, and if the particles of powder are overheated by the beam at the boundaries of the article, the sharp definition of the boundaries is lost. It is therefore essential that the particles of powder outside the defined boundaries of the article to be formed, to avoid being sintered and retain their individual particulate identities.
In the '817 patent, it is taught that a powder may be formed by blending plural materials having more than one bonding temperature, or, more than one dissociation temperature (col 4, lines 3-9). By "bonding temperature" is meant melting temperature or softening temperature. "Dissociation temperature" refers to the temperature at which a molecule breaks up into simpler portions.
Blending is illustrated by a blend of particles of first and second materials (see '817, FIG. 9) which blend is then heated in a portion of the sintering cycle (see col 6, lines 30-33) so that the second material (coating material) encapsulates the first material. The encapsulating material has a lower bonding or dissociation temperature than the first, so that, upon being further subjected to laser energy sufficient to produce sintering, the second material melts and infiltrates the powder mass in the area surrounding each particle of first material (see col 6, lines 39-42). In so doing, because the first material (before it is encapsulated) has a bonding temperature higher than that obtained with the laser during the sintering process, the second material (the coating material) melts but the first material is not sintered. It (first material) is held within in the heated mass by the infiltration of the coating (second material). In the final stages of sintering, the coating is dissociated and the virgin first material is sintered.
But the material with the higher bonding or dissociation temperature need not be sintered; it may retain its original structure (see '817, col 6, lines 53-57).
For example, a conducting material is coated with an insulating polymer material and sintered. The insulator is later removed, resulting in a conductive, sintered product. In another example, tungsten carbide particles may be coated with cobalt to produce a powder which when sintered, results in melting of the cobalt and infiltration of the tungsten carbide (see '817, top of col 7).
My invention requires that I use a sinterable host powder, and, that it be partially coated with a coating powder which preferably, is not sinterable at a temperature at which the sinterable powder is sintered. Further, the partially coated host powder cannot have a narrow range over which transition from one state to another occurs, but one which ranges over more than at least 3.degree. C. (referred to as a "broad range of transition"); and the coating powder cannot be dissociated. In my sintered article, interparticle chain diffusion occurs when partially coated, or mottled host particles are those of a synthetic resinous material (polymer); and, intermolecular diffusion occurs when the mottled host particles are not of an organic polymer.
Since I require either interparticle chain diffusion or intermolecular diffusion for sintering a host powder, it seemed that encapsulating a sinterable host material with a coating powder having a higher temperature than the host, was simply the wrong thing to do. Further, it seemed likely that, as long as some portion of the host powder material was uncoated, it would be unavoidably sintered at a temperature which was only slightly above (a) its glass transition temperature T.sub.g (if the host powder is amorphous), or, (b) its m pt (if the host powder is crystalline), or, (c) above its heat distortion temperature (HDT) if the host powder is a thermosetting resin ("TSR").
A crystalline host powder is an inapt choice for a solution to the problems to be solved in this invention, because a crystalline powder generally has a relatively sharp m p, and therefor does not offer an adequate "temperature window of sinterability" ("WOS" for brevity). A typical melting point extends over a range of less than about 3.degree. C., unless the powder has a high molecular weight, or contains impurities; or the powder, though pure, contains infinitesimal portions on a submicroscopic or molecular scale which have widely varying molecular weights, such as are found in many waxes. By a WOS (temperature window of sinterability) I refer to a range of temperature, from that at which laser-sintering commences at the initial T.sub.g of the coated powder, up to its final T.sub.g, at which sintering ceases. A temperature high enough that sintering ceases is evidenced by the coated powder becoming a semi-solid or liquid having essentially no porosity.
A powder of a TSR is also an ill-suited choice for the purpose at hand because the surfaces of individual TSR particles do not have acceptable fusion characteristics when heated by a laser beam. Therefore the desirable host powder of this invention is of a thermoplastic material. No further consideration will be accorded either crystalline host powders or powders of a TSR in this specification.
It should be recognized that, though all thermoplastic synthetic resinous powders can be sintered, the problem is to produce a sintered part of adequate strength for the purpose at hand. It is known that the higher the bulk density of the powder used, the higher is the density of the layers of the powder which are to be sintered, and the greater is the strength of the article formed. In this context, it was decided to choose a host powder known to be sinterable, and one which could be coated with a coating powder in a dry-blending process.
In the prior art, since the powder to be sintered was either not coated, or coated to lower its sintering temperature, it was not possible to maintain a target bed of powder above the initial T.sub.g of the powder without having the bed cake up.
The function of the coating powder was (a) to raise the storage temperature at which the host powder could be stored as much as possible without allowing the powder to cake, (b) to negate the expected static electrical charge due to mechanically dry-blending, and (c) to densify the coated powder, while the coating powder was to undergo no change in physical state. This function of the coating powder is the same whether it is crystalline such as in magnesium orthosilicate having a sharp melting point, or as in poly(butyleneterephthalate) (PBT), nylon 11, poly(phenylene sulfide) (PPS), or poly(ethyleneterephthalate) (PET); or, amorphous with a high T.sub.g such as a polyarylene polyether; or, amorphous in one temperature range and crystalline in another, as in a thermoplastic elastomer having a high m pt. In other words, other than its finely divided powdery state, which allows the coating powder to adhere to the surfaces of host particles without the coating particles being sintered, it is immaterial whether the composition of the coating powder is organic or inorganic, crystalline or not, as long as it undergoes no change in physical state. Therefore a coating powder is chosen in which the individual particles are as small as can be obtained, and which do not coalesce during sintering of the coated powder. A preferred coating powder has a particle size in the range from 0.02 .mu.to 1 .mu., more preferably &lt;0.2 .mu..
The T.sub.g of a powder is determined to be the midpoint of a range from the temperature of onset of transition (referred to as the "initial T.sub.g "), to the temperature of the completion of transition (referred to as the "final T.sub.g "). These initial and final points on a trace (see FIG. 3, described in greater detail below) generated by a differential scanning calorimeter ("DSC") provide evidence of the transitions which are a fundamental physical property of a material. The temperature at onset of transition is so closely related to the "caking temperature" that for all practical purposes, it is essentially the same.
It just so happened that a host powder, mottled by blending it with a chosen coating powder as specified herein, does not coalesce under conditions at which the virgin powder would. This characteristic allows a bed of the mottled particles to be stored at a temperature above the initial T.sub.g or "onset of transition" of the host powder without becoming lumpy due to particles sticking together. Prolonged storage of the coated powder is essential in both, the "feed" cylinder and also the "parts" cylinder of a SLS machine. Such storage is essential in the (i) feed cylinder, because in the best mode, the powder is transferred to the target surface of the parts cylinder at as high a temperature as is practical; and, (ii) parts cylinder because in the best mode of a sintering operation, the unsintered powder is recovered and reused without having become lumpy due to coalescence of coated particles in either bed of the SLS machine.
The temperature at which a sinterable powder can be stored is of particular interest in this invention because it has been found that the higher the storage temperature, the higher the strength of the sintered part in comparison with an identically sintered part of the same powder stored at a lower temperature. The upper limit of the storage temperature is that at which the powder is "sticky" rather than flowable, and cannot be transferred as a powder, and spread in the target area because the powder has "caked". This temperature is therefore referred to as the "caking temperature" or "coalescence temperature".
As will presently be evident, the storage temperature can be substantially higher than the temperature at which transition of the host powder is complete, because the host powder is surprisingly well-insulated by the partial coating of the coating powder, but not for very long. Further, the risk of caking increases quickly as the storage temperature rises above the final T.sub.g of the host material.
A mass of mottled particles is also unique because, after being blended at a temperature above ambient (20.degree. C.-30.degree. C.), the coated powder affords a larger WOS in a SLS machine than afforded by virgin powder.
The '816 patent teaches that "densifying" a sinterable powder is desirable, and various methods are taught as to how this might be accomplished. For example, a powder depositing device is taught which electrostatically charges and dispenses the powder to a target area having a charge opposite to that of the powder. Other examples are provided for densifying a powder of preselected particle size, after the powder is placed in the SLS machine for use.
There is no suggestion in either the '816 or '817 patents that one might start with a partially coated fine powder in which the coating does not melt, particularly a powder in which the bulk density has been increased relative to the powder's original bulk density (that of the powder when produced by the manufacturer), before the powder is placed in the SLS machine; nor is there a suggestion as to how one might maintain the particulate free-flowing condition of the powder, regardless of its bulk density, at or near, much less above, the caking temperature or initial T.sub.g.
The problem of distributing a powder with a proclivity to "cake" or "become sticky" was not encountered, and therefore not addressed, in either the '816 or the '817 patents. Referring specifically to the former, it will be appreciated that the powder must be flowable under applied force, if it is to be applied as a layer by using a drum which rotates counter to the direction of its movement across a target area upon which the layer is to be deposited. If the powder "cakes" it becomes lumpy and cannot be spread in a layer less than 80 mesh (about 180 .mu. thick), of relatively uniform thickness, such as is necessary to form a selectively laser sintered article.
It would have been logical to add an anti-caking agent, if caking was a problem at a temperature below the initial T.sub.g of the host powder. The function of the anti-caking agent would be self-evident. To prevent the host powder from caking at a temperature above its initial T.sub.g, it would be logical to encapsulate the host powder with a coating powder which has a higher T.sub.g than the host. Quite unexpectedly however, when the host powder is only partially coated with the coating powder, and even when coated over only a minor portion of the surface area of the particles of host powder, the coated host powder does not cake above its initial T.sub.g when stored for a prolonged period.
No less unexpectedly, in my invention, the coating powder also aids in densifying the coated powder, hence functions as a "densifier" provided the coating powder is blended with the host powder under high-shear conditions and discharged at a temperature, referred to as the "drop" temperature, in the range from within 20.degree. C. below its initial T.sub.g, up to a temperature lower than, but near the final T.sub.g of the host powder, preferably within 2.degree. C. of the final T.sub.g. The "drop temperature" is the recorded temperature of the coated powder mass at the end of the mixing period, just prior to the mass being dumped from the mixer, and reflects the energy input.
Such densification occurs mainly because the blended partially coated powder, though not substantially diminished in average particle size, is essentially free of electrical charge.
Since the true specific gravity of most polymer powders is in the range from about 0.6-1.5 g/cc, the bulk density of a polymer powder, even in pellets about 3 mm in avg size, seldom exceeds 1.0 gm/cc. Typically, this is the range of bulk density of the powder feed to an injection molding machine or an extruder, whether the powder is in a size range greater than about 80 mesh, or in the form of pellets as large as about 4 mesh in size, or even larger. Fine polymer powders, that is, no greater than about 177 .mu. in average particle size, are not used in either injection molding machines or extruders because there is no advantage to offset the penalty of the much lower bulk density which is typically associated with the fine powder.
The typical host powder &lt;177 .mu. has a bulk density in the range from 0.3 to 0.6 gm/cc depending upon the true density of the material and the shapes of the particles. Commercially available PVC powder when comminuted typically has a bulk density in the range from 0.35 to 0.6 gm/cm.sup.3 ; and PC ranges from 0.33 to 0.55 gm/cm.sup.3.
From the foregoing it will now be evident that the partially coated powder of this invention is specifically tailored for use in a SLS machine when it is operated by following steps consistent with those outlined hereinafter.
It is self-evident that `fines` can increase the bulk density of a polymer powder by filling interstices between the coarser particles. It is also known that fines can actually increase the bulk density of resins in a compacted state, if the necessary compaction is not frustrated by static charge effects. It is equally self-evident that it will not be efficient to feed a fluffy mass of very fine particles to the target area of an SLS machine, just as it would be inefficient for an injection molding machine or extruder because of bridging, air entrapped in the powder, and still other reasons.
Yet, it is efficient to feed very fine and fluffy particles (e.g. Hi-Sil) for the specific occasion when it is desirable to provide a feed of thoroughly mixed powders in a mass in which the host particle size is in the range from 1 .mu.-177 .mu., for the purpose of forming a laser-sintered article without sacrificing the relatively high bulk density of a mixture of the coarse powders (because of the mixture's ability to self-pack densely), or, its good particulate flow characteristics.
The prior art relating to finely divided powders, having particles &lt;180 .mu., being blended for the specific purpose of densifying the powder, is strangely silent as to how this might be accomplished. Pellets and very coarse powder have been reduced to a smaller particle size by shearing the solid particles at low shear, in a mixer maintained at elevated temperature. But the bulk density of the comminuted, prior art particulate mass was typically decreased and not increased because the input of mixing energy at low shear was insufficient. By "low shear" I refer to an energy input of less than about 30 watt-hr per lb of powder, but preferably in excess of about 1 watt-hr/lb of powder, a greater energy input than the former (30 watt-hr/lb of powder) being referred to as "high shear". It will be appreciated that the precise quantum of energy required to provide "high shear" mixing of powders is not quantified critically, but will depend to some extent upon the particular sizes of powder mixed, and also the morphological characteristics of their compositions; and, that slightly less than high shear conditions may provide some, but not optimum densification.
By and large, one skilled in the art, faced with the problem of providing a dense, fine powder of a coated polymer essentially free of static electric charge and having a bulk density which is at least as high as that of the virgin polymer in a coarser particle size, resorts to crystallizing, precipitating, or otherwise re-conditioning the coarse powder into as fine a powder as will not significantly decrease its bulk density from before it was coated. One simply does not comminute the coarse powder with an anti-caking or densifying coating powder, with the expectation that the resulting finer, partially coated powder will substantially maintain the bulk density of the starting coarse powder and be free of electric charge.
The reason is that most polymer powders are essentially non-conductive, and comminution of the powder by any comminuting means, particularly a metal blade means, generates a significant level of a static charge which affects both the flow and the packing characteristics of the powder. Static is a dynamic property. Static is generated during the movement of powders, for example, as it is being discharged from a conduit. The rate at which the static is generated is dependent upon the conductivity of the powder and the grounding of the system.
Therefore the performance properties of powders, and in particular, their resistance to caking and self-packing, both of which are affected by static, are also dependent upon their history and the inherent properties of the powders themselves, particularly particle size, shape and uniformity. See "The Powder Properties of Poly(vinyl chloride) Resins" by Paul R. Schwaegerle, ANTEC '84, p 824. Conditioning the powder at a relative humidity of 50% or greater, effectively countered and negated the static charge provided the powders were allowed enough time to recover their original flow and packing characteristics. No other suggestion was made as to how to negate the effects of the static charge build-up.
The facility with which one may maintain a coated/uncoated powder flowable under applied force, which powder does not exhibit caking, and without decreasing the bulk density of the partially coated powder after it has been comminuted, is progressively exacerbated if a component of the coated powder is organic, and the coated powder is then to be maintained at elevated temperature, particularly one approaching or exceeding the initial T.sub.g or caking temperature of the host material, but below its final T.sub.g.
Because a coarse powder (or pellets) having relatively high bulk density is typically free-flowing, there is no reason to coat either the coarse powder or the pellets with an anti-caking agent, or otherwise coat or encapsulate them. But the desirability of a fine powder having relatively high bulk density, yet which flows under applied force, and is resistant to caking at or above the initial T.sub.g of the powder, is of utmost interest in a method for sintering the powder with a laser to produce as dense a porous article of arbitrary shape as can be made.